This invention relates to multiple layer sheet materials, and especially multiple layer packaging sheet materials which are used to fabricate packages through the formation of heat seals about the periphery of the package. The invention is particularly related to those films which use a layer of vinylidene chloride copolymer as an interior layer in the sheet materials in combination with a layer of a polymeric composition which typically has a softening temperature similar to, or higher than, that of the vinylidene chloride copolymer in an exterior layer of the film. The exterior layer is adapted for use in the formation of heat seals.
In forming heat seals in such sheet materials, the amount of heat which is driven through the sheet material to the sealant layer to soften that layer sufficiently for formation of the heat seals, is also, as a secondary and undesirable function, sufficiently intense to at least somewhat soften the vinylidene chloride copolymer layer during the formation of the heat seals.
Packages made with conventional ones of such sheet materials, to the extent the overall thickness of the sheet material is less than about 0.38 mm., tend to be deficient in impact tolerance; especially packages where such sheet materials are used as closure lids on formed trays. In such applications, the lid is the weakest member of the package, such that any failure of the package typically occurs in the lid sheet material adjacent the heat seal.
"Softening temperature" as used herein is any determinant and measurable temperature which identifies a condition at which the polymer experiences a change which tends to make it more fluid, and subject to flowing at normal conditions used in forming heat seals. While the DSC melting point is generally referred to herein, other tests could equally well be used so long as they are applied equally to all the layers being compared.
It is an object of this invention to provide improved multiple layer sheet material having an interior layer of a vinylidene chloride copolymer in combination with a sealant layer, and wherein there is provided an impact layer having elastomeric properties and improved impact tolerance in the resulting sheet material, such that the impact tolerance of the package is improved.
It is especially an object to provide increased impact tolerance at the heat seal locus, while maintaining the ability, in the sheet materials, to form heat seals sufficiently strong to maintain the integrity of the seal throughout the intended use life of the package until it is intentionally opened.
It is yet another object of the invention to provide such films without significantly increasing the amount, or the cost, of the materials used, or the overall thickness of the films.